Membrane for use in dialysis and ultrafiltration and the production of such member

ABSTRACT

This invention relates to a membrane suitable for use in dialysis processes and ultrafiltration processes to separate, concentrate or purify a substance of a molecular weight of 500 to 20,000, heretofore difficult to be treated by the various known types of dialysis membranes or ultrafiltration membranes. The membrane of the invention is characterized by its higher water flow rate and by the anisotropy of its structure made of an ionically cross-linked polymer of a polycation with a polyanion; the structure comprising a continuous and dense microporous top surface layer with an estimated averate pore diameter of about 10 to 120 angstroms and a lower, integral microporous reinforcing layer containing micropores of larger pore diameters, said membrane being supported by a porous layer of a polyolefin or polyamide. The membrane may be produced by applying to a porous supporting sheet a thin liquid film of an aqueous-organic solution containing both a polycation and a polyanion together with an inorganic sodium or calcium salt, then treating the film so as to increase the dielectric constant value in its top surface, washing the film in a water bath under controlled conditions and repeating operations of subsequent rinsing and drying the film.

Hashimoto et al.

June 5, 1973 MEMBRANE FOR USE IN DIALYSIS AND ULTRAFILTRATION AND THEPRODUCTION OF SUCH MEMBER Inventors: Koichi Hashimoto, Tokyo; HidekoKishida, Kashihara, both of Japan Assignee: Uivac Corporation,Chigasaki-shi,

Kanagawa-ken, Japan Filed: July 8, 1970 Appl. No.: 53,221

Foreign Application Priority Data Dec. 20, 1969 Japan ..44/1021l8 US.Cl. ..2l0/490, 210/500, 264/49 Int. Cl. ..B0ld 31/00, B01d 13/00 Fieldof Search ..264/49; 210/22, 23, 210/321, 500, 507, 490

References Cited UNITED STATES PATENTS Primary Examiner-Frank A. Spear,Jr. Attorney-Ely, Golrick & Flynn [57] ABSTRACT This invention relatesto a membrane suitable for use in dialysis processes and ultrafiltrationprocesses to separate, concentrate or purify a substance of a molecularweight of 500 to 20,000, heretofore difficult to be treated by thevarious known types of dialysis membranes or ultrafiltration membranes.The membrane of the invention is characterized by its higher water flowrate and by the anisotropy of its structure made of an ionicallycross-linked polymer of a polycation with a polyanion; the structurecomprising a continuous and dense microporous top surface layer with anestimated averate pore diameter of about 10 to 120 angstroms and alower, integral microporous reinforcing layer containing micropores oflarger pore diameters, said membrane being supported by a porous layerof a polyolefin or polyamide. The membrane may be produced by applyingto a porous supporting sheet a thin liquid film of an aqueous-organicsolution containing both a polycation and a polyanion together with aninorganic sodium or calcium salt, then treating the film so as toincrease the dielectric constant value in its top surface, washing thefilm in a water bath under controlled conditions and repeatingoperations of subsequent rinsing and drying,

the film.

27 Claims, 1 Drawing Figure (Approx. 5600x) H PATENTEDJUE' 5 I975Approx. 5600x) INVENTORS HASHIMOTO & HIDEKO KISHIDA KOICHI ATTYS.

MEMBRANE FOR USE IN DIALYSIS AND ULTRAFILTRATION AND THE PRODUCTION OFSUCH MEMBER This invention relates to a new type of membrane for use indialysis and ultrafiltration processes and by means of which a solutioncontaining a substance hav- -ing a molecular weight in a range of about500 to 20,000 may be dialysed or ultrafiltered to effect an efficientand rapid separation, concentration or purification of said substance byknown usual processes of dialysis or ultrafiltration. The presentinvention further relates to the production of such membrane from asolution containing oppositely charged polyelectrolytes dissolved in anaqueous organic solvent system of a controlled composition containing aninorganic salt.

The bodies of known types of ultrafiltration membranes have usuallycontained micro-pores the attainable minimum diameter of which in theprior art amounts only to about m namely 200A. Now a molecule having amolecular weight on the order of 1,000,000 shows a diameter ofapproximately 200A., assuming that it is of a substantially sphericalconfiguration. Consequently, it is not possible to utilize the knownmembranes for ultrafiltering a solution containing substances ofmolecular weights of less than 1,000,000, since such substances canfreely pass through such micro-pores.

It is also known that cellophane membranes and collodion membraneshitherto available as dialysis membranes can only be used to effect thedialysis of a solution containing a substance having a molecular weightin a range of a few thousands to a few tens of thousands, for it has notbeen possible to control and predetermine the diameter range of themicro-pores in the course of manufacture. Accordingly it has beennecessary to subject these membranes as produced to a series of tests inorder to determine the range of molecular weights of substances whichmay effectively be dialysed therewith. Besides, the known cellophanedialysis membranes have usually shown a low rate of permeation for waterand hence have been less efficient in practice.

Moreover, where a cellulose acetate membrane is used for reverseosmosis, the substances which may be separated, concentrated or purifiedhave been limited to those having low molecular weights, such as sodiumchloride. With the known cellulose acetate reverse osmosis membranes,there are similar problems of infeasibility of controlling andpredetermining pore diameters during manufacture, and necessity oftesting, as above described. Also the known cellulose acetate membranefurther disadvantageously usually exhibits a very much lower rate ofwater diffusion or permeation than the ordinary cellophane dialysismembranes. Besides, the reverse osmosis is technically inconvenient inthat it essentially requires an application of an extremely highpressure on the one side of the membrane in order to overcome theosmotic pressure.

It has been proposed that, as the dialysis membrane, there be used asolid film of an ionically cross-linked -polymeric reaction product,formed by contacting an aqueous solution containing a suitableconcentration of a polyelectrolyte having polymeric anionic groups,(such as a polymer of sodium styrene sulfonate) with an aqueous solutioncontaining a suitable concentration of a polyelectrolyte havingpolymeric cationic groups (such as a polymer or vinylbenzyl trimethylammonium chloride) within the body or on the surface of a porous supportsheet, so that the polyelectrolytes react and ionically cross-link atthe interface where the solutions come into contact with each other,whereby a solid film of said ionically cross linked polymeric reactionproduct is deposited at said interface (see US. Pat. No. 3,276,598).Although this known type of supported dialysis membrane has a highpermeability toward simple substances of lower molecular weight such assodium chloride, it haspractically no permeability toward highermolecular weight compounds. Accordingly, this last-mentioned membrane isnot suitable for dialysis of a solution containing a first substancehaving a molecular weight in a range of 500 to 20,000 together withanother compound or compounds having molecular weight(s) near to orwithin the range of 500 to 20,000, for the purpose of separating,concentrating or purifying the first substance free from said othercompound(s). Besides, it is not feasible to control and predetermine thediameter of the micro-pores present in the polymeric film of this kindof dialysis membrane in the course of manufacture.

An object of the present invention is to provide a new type of membranefor use in the ultrafiltration and dialysis processes which is not onlycapable of exhibiting a water permeation rate of about 10 to 1,000 timeshigher than those of the prior available ultrafiltration, dialysis andreverse osmosis membranes, but which also is capable of separating,concentrating or purifying a substance having a molecular weight in arange on the order of 500 to 20,000. Another object is provision of thedescribed membrane wherein a top layer contains active micro-pores ofrelatively uniform diameter and an average diameter adjustable in arange of about 10 to A. Another object of the present invention is toprovide a new type of membrane which may be so manufactured that themolecular weight of substances to be treated therewith may be optionallypre-determined and controlled during membrane manufacture. A furtherobject is to provide a highly reproducible process for the production ofa membrane exhibiting the abovementioned favorable properties when usedas an ultrafiltration or dialysis membrane.

It is known that when an aqueous solution of an acidic or anionicpolyelectrolyte is'merely mixed with an aqueous solution of a basic orcationic polyelectrolyte, the cationically and anionically dissociatedgroups in both the dissolved polyelectrolytes undergo the ionicinteraction and linkage, so that both polyelectrolytes immediatelycombine with each other and can precipitate in the form of clumps. We,the inventors, have now found that a solvent system, prepared bydissolving at least one of sodium bromide, sodium iodide, calciumchloride, calcium bromide and calcium nitrate at a concentration of 10to 35 percent by weight (based on the weight of the resulting solventsystem) into a mixture of acetone and water or a mixture of dioxane andwater (at a mixing ratio such as to give a dielectric constant value of50 to 78 as determined at 20C.), is able to dissolve and containsimultaneously, either in a salt form or in the free state, both awater-soluble anionic polyelectrolyte carrying sulfonic acid groups andalso a water-soluble cationic polyelectrolyte carrying quaternaryammonium groups or pyridinium groups, while permitting saidpolyelectrolytes to remain dissolved in said solvent system in the formof a solution (more exactly speaking, in the sol solution state) withoutbringing about the coprecipitation which otherwise would take place byionic interaction of these oppositely charged polyelectrolytes.

We have further discovered very interesting behavior displayed by such asolution of the oppositely charged polyelectrolytes which has beenprepared by dissolving in the above-mentioned solvent system, to a totalcontent of 2-8 percent by weight based on the quantity of the solventsystem, a sulfonic acid group-carrying polyelectrolyte, or its salt,having a molecular weight of at least 50,000 and containing a sufficientnumber of the sulfonic acid groups to 'give a pH of 2 or less when saidpolyelectrolyte, in the form of the free acid, is dissolved in water toa concentration of about 06-54 percent by weight, as well as aquaternary ammonium groupcarrying or pyridinium group-carryingpolyelectrolyte or its salt exhibiting a molecular weight of at least50,000 and containing a sufficient number of the quaternary ammoniumgroups or pyridinium groups to give a pH of 10 or more when saidquaternary ammonium groupor pyridinium group-carrying polyelectrolyte,in the form of the free base, is dissolved in water to a concentrationof about 0.6-5.4 percent by weight. Thus, we have found that when the soprepared solution is spread or cast into a thin liquid film at aconstant temperature by application on a suitable support and then thevalue of the dielectric constant of the solution in the top surfacelayer of the thin liquid layer is increased by a suitable means, the topsurface layer can be gelled; and that when the liquid film with thegelled surface layer is subsequently immersed in a bath of water,whereby the existing inorganic electrolytes and organic solvent areremoved from the whole film into the water bath, a firmly bondedthree-dimensional network of the ionically cross-linked polymer isformed within the body of the film, giving an anisotropic structurecomprising a dense top layer and a coarse lower layer. When across-section across the solidified film is exam ined under an electronmicroscope, it has been observed that the top surface layer of the solidfilm has an electro-microscopically continuous and dense structure,whereas the lower layer has a micro-porous but rather sponge-likestructure. In addition, it has been found that this continuous, topsurface layer exhibits a sieving ability in allowing selectivelysubstances of smaller molecular weights than a certain molecular weightto permeate therethrough but preventing substances of larger molecularweights from passing therethrough.

It also has been found that by choosing suitably the natures, molecularweights and concentrations of the polyelectrolytes used, the natures andnumbers of the ionically dissociable groups of the polyelectrolytes, thenature and concentrations of the inorganic salt used etc., as well asthe ratio of the organic solvent to water, and by selectingappropriately the process conditions, especially for gelation andwater-immersion during the formation of the gelled and solidified filmof the crosslinked polyelectrolyte polymer, the continuous, top surfacelayer may be produced with a very finely microporous structure which 1.exhibits a considerably higher water-permeation rate,

2. may be estimated to contain micro-pores of uniform size and of acontrolled average pore diameter in a range of about 10 to lA., and

3. is useful to separate, concentrate or purify a substance having amolecular weight in a range on the order of 500 to 20,000 by thedialysis or ultrafiltration processes.

The present invention is based on these aforementioned discoveries.

According to a first aspect of the present invention, therefore, thereis provided a membrane adapted to be used in the ultrafiltration anddialysis processes, characterized as comprising:

a. a diffusive and continuous top surface layer, on the order of 1 inthickness, formed of a polymer produced by ionically cross-linking, at aratio of equivalents of from 2:1 to 1.2, a water-soluble, acidicpolyelectrolyte or a salt thereof, with a water-soluble basicpolyelectrolyte or a salt thereof; the polyelectrolytes each having amolecular weight of 50,000-700,000; said acid polyelectrolyte containinga sufficient number, e.g., of sulfonic acid groups, to give a pH of upto 2 where said acidic polyelectrolyte is dissolved in the form of itsfree acid at a concentration of 0.65.4 percent by weight in water, andsaid basic polyelectrolyte containing a sufficient number of quaternaryammonium groups or pyridinium groups to give a pH of at least 10 wheresaid basic polyelectrolyte is dissolved in the form of its free base ata concentration of 0.6-5 .4 percent by weight in water; said top surfacelayer showing a dense and continuous structure as observed under anelectron microscope at 5,600X magnification;

b. a reinforcing intermediate layer formed of the same ionicallycross-linked polymer as, and integral with, said continuous top surfacelayer, and exhibiting a micro-porous but rather sponge-like structure asobserved under the electron microscope at 5,600X magnification; and

c. a porous support to which is tightly adhering the material of theintermediate layer; the support being formed of a polymer selected frompolypropylenes and polyamides and exhibiting a porous structurecontaining pores of an average pore diameter of up to 10p. but showing awater-flow rate of higher than 0.5 mLIcm'lmin. as determined with waterat 20C. and at a pressure difference of 760mm Hg.

According to a second aspect of the present invention, for theproduction of such a membrane as defined above, there is furtherprovided a process characterized as comprising:

i. preparing a solvent system by dissolving at least one of sodiumbromide, sodium iodide, calcium chloride, calcium bromide and calciumnitrate, to a concentration of 10-35 percent by weight of the weight ofthe resulting solvent system, into a mixture of acetone and water orinto a mixture of dioxane and water mixed together at such 'a ratio asto give a dielectric constant value of from 50 to 78 when determined at20C;

ii. dissolving into said solvent system, to a total concentration of 2-8percent by weight based on the quantity of the solvent system, at aratio of 2:l to 1:2 by equivalents, a water-soluble acidicpolyelectrolyte (or a salt thereof) and a water-soluble basicpolyelectrolyte (or a salt thereof), said polyelectrolytes (and salts)being such as described in the immediately preceding paragraph;

iii. spreading the resulting solution or sol solution of thepolyelectrolytes as a thin and uniform liquid film onto a support sheetmade of a polypropylene or polyamide as characterized in the precedingparagraph;

iv. treating the thin liquid film at a temperature of from to 50C. so asto increase the dielectric constant value of the solvent system in thetop surface layer of said thin liquid film;

v. continuing this treatment for a period of 5-120 minutes to bringabout gelation in the liquid film;

vi. immersing the gelled liquid film together with the support sheet ina bath of water at a temperature of from 10 to 50C. for a period of 5 to60 minutes to remove at least parts of the inorganic salt and theacetone or dioxane from the gelled film;

vii. withdrawing the solidified film together with the support from thebath,

viii. drying the assembly or composite of the film and support at atemperature of 50 to 70C. for a period of 3 to 30 minutes;

ix. then rinsing said composite with water at 10 to 50C. for a period of5 to 60 minutes; and

x. subsequently repeating the cycle of the above drying and rinsingoperations until the solidified-film on the support is completely freefrom inorganic salts and the organic solvent, namely acetone or dioxane.

By the above-used description or characterization capable of separating,concentrating or purifying a substance having a molecular weight in arange of the order of 500 to 20,000," it is meant that when the membraneis used in a known manner to ultrafilter or dialyse a solutioncontaining a desired substance together with any other compound(s)having molecular weight(s) lower than, or at times higher than, that ofthe desired substance, said membrane is able to separate, concentrate orpurify the desired substance, free from the other compound(s) of thelower molecular weight(s) by permitting all or substantially all of themolecules of said other compound(s) of the lower molecular weight(s) andsome molecules of the solvent to pass through the membrane to thefiltrate side or the liquid side external to the membrane, whilepreventing all or substantially all of the molecules of said desiredsubstance together with any such other compound of the higher molecularweight from passing to the filtrate side or external liquid side.Conversely that characterization may also mean that, when the membraneis used to ultrafilter a solution containing a desired substancetogether with any other compound(s) having molecular weight(s) higherthan, and occasionally also lower than, that of the desired substance,said membrane is able to separate, concentrate or purify the desiredsubstance free from the other compound(s) of the higher Imolecular-weight(s) by permitting all or substantially all of themolecules of the desired substance and the other compound(s) of thelower molecular weight(s) and some molecules of the solvent to penetratethrough the membrane into the filtrate side external to said membrane,while preventing all or substantially all the molecules of the othercompound(s) of the higher molecular weight(s) from passing through themembrane to the filtrate side. It is to be understood, however, that theabove-quoted description does not necessarily mean that the desiredsubstance has been completely freed from the molecules of the solventused to prepare the solution of said desired substance, because it isusual to practice that the dialysis and even the in be not forced toproceed until the solvent has entirely been removed from the solution ofthe desired substance treated.

The membrane of the present invention may generally be used in anymanner known per se for the ultrafiltration or the dialysis, that is tosay, either with the membrane positioned in contact with a solution ofthe substance to be treated with or without applying a pressure on saidsolution, so that the membrane serves merely as a filter for saidsolution, or with the membrane positioned in contact with and betweenthe phase of the solution of the substance to be treated and the purephase of the solvent used to prepare said solution, so that the membranecan act to dialyse said solution. Accordingly, the membrane of thepresent invention may be called either an ultrafiltration membrane or adialysis membrane, depending on the actual use.

The solution containing the substance to be treated may be prepared withan appropriate inorganic or organic solvent, e.g., water, alcohols,ketones, hydrocarbons etc., in which the cross-linked polyelectrolytematerial constituting the membrane is insoluble.

Whether used as an ultrafiltration membrane or as a dialysis membrane,the micro-pores of a given average pore diameter present in the toplayer act like meshes in a sieve. Thus, a solution to be treated,containing the desired substance and also contaminating or undesiredcompound(s) having different molecular weights and hence differentmolecular sizes, may microscopically be regarded as a mixture of variousmolecules of different molecular sizes. When this mixture isultrafiltered or dialyzed by means of the present membrane it can beseparated into two fractions; one fraction comprising molecules havingmolecular sizes equal to and larger than the minimum diameter of themicro-pores of the top layer and thus retained on the membrane, and theother fraction comprising the smaller molecular sizes permitted to pass,through the micro-pores, from one side to the opposite side of themembrane. This may be called the molecular-sieving effect of themembrane. Either may be collected or recovered as a desired fraction.Under these circumstances, we may designate the membrane of the presentinvention as a molecular-sieve membrane for a convenience of expressingcollectively all the functions of the membrane which are effective toseparate, concentrate or purify a substance.

Suitable examples of the acidic polyelectrolyte include polystyrenesulfonic acid, polyvinyl sulfonic acid, and their alkali or alkalineearth metal salts, particularly, the sodium and calcium salts. Aco-polymer of a styrene sulfonic acid or a vinyl sulfonic acid or analkali metal salt thereof with a copolymerizable monomer, such as, e.g.,styrene, may also be used as the watersoluble, acidic polyelectrolyte.

Suitable examples of the basic polyelectrolyte containing the quaternaryammonium groups include polyvinylbenzyl trimethyl ammonium hydroxide orits halides, particularly the chloride, as well as a co polymer ofvinylbenzyl trimethyl ammonium halide with a copolymerizable monomer.Further, the examples of the water-soluble, basic polyelectrolytecontaining the pyridinium groups include polyvinyl methyl pyridiniumhydroxide and its halides, particularly the chloride, as well as aco-polymer of vinyl methyl pyridinium halide with a copolymerizablemonomer such as styrene.

The acidic and basic polyelectrolytes must normally be solid and capableof film formation, and therefore it is necessary that each should have asufficiently high molecular weight of at least 50,000 and preferably ofat least 100,000. Those having a molecular weight of lower than 50,000cannot be used as their film-forming ability is poor. n the other hand,it is actually difficult to synthetize the polyelectrolytes of amolecular weight in excess of 700,000, and hence it is necessary thatthe molecular weight should be in the range of 50,000 to 700,000. Thepolyelectrolytes used may be either of straight chain or of branchedchain type, as long as they are soluble in water. It is preferable thatthe acidic and basic polyelectrolytes used should carry respectively atleast one sulfonic acid group or at least one quaternary ammonium orpyridinium group per three of the repeating monomeric units whichconstitute the polyelectrolyte molecules.' If an acidic polyelectrolyteof such an acidity as to give a pH exceeding the stipulated upper limitpH of 2 is ionically cross-linked with a basic polyelectrolyte of such abasicity as gives a pH of less than the lower limit pH of 10, it isdifficult to form the desired diffusive and continuous top surface layerand the sponge-like intermediate reinforcing layer in the film, becauseonly a weak ionic linkage force can be achieved between the ionic polygroups. Even if the continuous top surface layer and intermediatereinforcing layer can be formed in the film, these layers would be of alower mechanical strength and the permeation rate of water passingthrough them would be too low to make the membrane product practicablyefficient for the dialysis and ultrafiltration processes.

Although in the present invention it is preferred that the acidic andbasic polyelectrolytes be used in the ratio of 1:1 by equivalents toproduce a membrane which as a whole is neutral in nature, they may beused at an equivalent ratio falling in a range of 1:2 to 2:1 at maximumand preferably in a range of 121.5 to 1.521. In these cases, theresulting basic or acidic membrane will be useful for certain specialpurposes, as it has a higher ability to adsorb water than that of theneutral membrane.

The diffusive continuous top surface layer, having the primary activerole in the membrane, should be as thin as possible in order to give awater-permeation rate as high as possible, and thus it should preferablyhave a thickness on the order of 1 1., namely 10,000A. or less. Itappears microscopically that within this top layer several, at most 20,molecules of the ionically cross-linked polymer are overlapped in theform of strata, forming a three-dimensional network. In addition, it maybe estimated that this continuous top sur face layer contains very manyfine micro-pores of uniform diameter and interconnecting with each otheracross this layer, although these micro-pores cannot be observed and thestructure of this layer merely exhibits denseness or solidness andcontinuity as viewed even under the electron microscope at 5,600 to20,000X magnifications.

The intermediate reinforcing layer not only serves integrally to supportand reinforce the upper continuous top surface layer, but it also.tightly adheres to the low-most support, thereby serving to anchor thetop layer to the support. The intermediate reinforcing layer alsoprovides a cushioning action which presents the pressure or forceapplied to the membrane during operation or handling from being directlytransmitted to the very thin and relatively fragile top layer. It isnecessary that the intermediate layer have an appropriate porosity andthickness, such that it cannot prevent the passage of water or thecompound(s) which have passed through the top layer. It has been foundthat the intermediate layer should preferably be on the order of about0.1-0.2mm. in thickness. microscopic observation, it has been estimatedthat the intermediate reinforcing layer nonnally contains micro-pores ofabout 10 to mp. diameter.

The continuous top surface layer and the intermediate reinforcing layerhave a total thickness on the order of only 0.1 to 0.2mm. and, to imparta sufficiently practicable mechanical strength, they are fixed on aseparate support. It is necessary that this support be made of amaterial, e.g., polypropylenes or polyamides, compatible with and highlyadhesive to the material of the intermediate reinforcing layer. Amongthe available polyamides, polycaprolactams (namely 6-nylon) andpolyhexamethylene adipamides (namely 6,6-nylon)'are preferred, as thesesynthetic polymers are strongly adhesive to the above-mentionedcross-linked acidic and basic polyelectrolytes.

The support should exhibit a porous structure with a water-flow rate of10 or more times higher than the water permeation rate of the continuoustop surface layer, and hence of at least 0.5ml./cm. /min. as determinedwith water at 20C. under a pressure difference of 760 mm Hg, becauseotherwise the whole body of the membrane would show a lowerwater-permeation rate, and the throughput of the membrane would be toolow to be practicable. On the other hand, if the support material haspores of an excessive pore diameter, a breakdown can take placeultimately in the active top surface layer by the intermediatereinforcing layer and finally the continuous top surface layercollapsing into the pores of the support layer, due to the fact thatthese layers are subjected to the pressure applied to the membraneduring its operation. Accordingly it is required that the maximumaverage pore diameter in the support should be limited to about 10p.

It is preferable that the support be a flat sheet or mat of about 0.1 to2mm. thickness, prepared by using, as the substrate material, the fibersor filaments of one of the above-mentioned synthetic polymers, formed toa non-woven cloth with application of elevated temperature and a highrolling pressure, thereby smoothing the cloth surfaces andsimultaneously affording the porous structure of properties which 'meetthe abovementioned requirements. The support sheet may also be in theshape of a tube or cylinder or bag, if desired. Thus, the membrane ofthe present invention may be formed into a bag or may be held by asupporting frame for use in the ultrafiltration or dialysis processes.

Referring to the accompanying drawing:

The FIGURE shows an electron microscope photomicrograph of thecross-section through the continuous top surface layer and theintermediate reinforcing layer of a membrane product according to thepresent invention, at a magnification of about 5,600.

In the production of the molecular-sieve membrane according to thepresent process, the inorganic salt which is incorporated into thesolvent system should be restricted to sodium bromide, sodium iodide,calcium chloride, calcium bromide and calcium nitrate. From ourexperiments it has been found that the objects of the present inventioncannot be achieved if other inorganic salts are employed. Althoughsodium bromide is most preferable, it is possible to select any suitableone or more salts from the above-mentioned class, depend- Fromelectroning on the natures of the acidic and basic polyelectrolytes usedand depending on the average pore diameter of the micro-pores which areto be produced in the continuous top surface layer. Where one of suchinorganic salts has been dissolved in the solvent system, it usuallypromotes the ionic dissociation of the acidic and basic polyelectrolytessubsequently introduced. It is known that the cations and anions derivedfrom the inorganic electrolyte then become positioned between themolecules of the acidic and of the basic polyelectrolytes which areionically dissociated in the same solution, whereby these oppositelycharged polyelectrolytes can be prevented from coprecipating asotherwise would take place due to their ionic interaction, and wherebythe solubilities of these polyelectrolytes may rather be increased.

In cooperation with the known action of the inorganic salt, the factthat the acetone-water mixture or the dioxane-water mixture constitutingthe solvent system has a dielectric constant of 50 to 78 is useful toinsure that the oppositely charged polyelectrolytes are maintained insolution in the sol state and so prevented from the co-precipitation.The quantity of the inorganic salt may be selected to give anappropriate concentration depending on the actual value of the dielectric constant of the solvent system used, and on the nature of and theactual ionic strength values of the polyelectrolytes. Moreover, thevalue of the dielectric constant for the acetone-water mixture or thedioxanewater mixture may be selected to be appropriate in the range of50 to 78 depending on the nature and the concentration of the inorganicsalt used, and the natures and the ionic strengths of thepolyelectrolytes used. An optimum value of the dielectric constant andalso an optimum quantity of the salt to be used may readily be decidedby carrying out preliminary experiments. If desired, dimethyl formamideor glycerine may be present in the acetone-water mixture or thedioxanewater mixture to a concentration of up to percent by weight ofthe whole mixture.

If the total polyelectrolyte concentration is greater than the upperlimit of the above-defined 2 to 8 percent range, the resulting solutionis then too viscous to be spread or cast into a uniform and thin liquidfilm on the support; but if less than the lower limit, the solidifiedfilm is then too weak to be practicable.

In the present process, the described solution of polyelectrolytes isspread or castinto a thin liquid film on a support sheet in aconventional manner, for instance, in such a way that the support sheetis placed under tension on a flat glass plate or metal panel and thespreading solution applied uniformly on the top side of the supportsheet by means of a conventional device such as doctor blade orroller.

From our experiments it has been found that about 0.15 to 0.4ml./cm. ofthe film-forming solution is usually necessary to be applied to give athickness of about 0.1 to 0.2mm. for the continuous top surface layerand the intermediate reinforcing layer. The liquid film applied willgradually be reduced in thickness during the subsequent steps. The thinliquid film so spread on the support sheet is then adjusted to be at atemperature of 10 to 50C. and subsequently subjected to the treatment bywhich the dielectric constant value of the solvent system in the topsurface layer of the applied film is increased while keeping said filmat a temperature in said range of 10-50C.

The treatment for increasing the dielectric constant in the top surfacelayer of the liquid film may be performed either by evaporating theacetone or dioxane out of the surface by letting the supported liquidfilm stand in air at 10 to 50C. for a period of 5 to minutes, or byblowing a cold and inert gas, for example, nitrogen gas or air at about5C. onto the top surface to lower the temperature in the top surfacelayer, or by blowing dimethyl formamide vapor onto the top surface ofthe liquid film. When this treatment is continued for a period of 5-120minutes, the cast liquid film can be gelled. If theperiod of thistreatment is shorter than 5 minutes, the gelation of the liquid filmcannot proceed completely, so that a satisfactory membrane is notproduced; while if longer than 120 minutes, cracks or pin holes orlarger holes can be formed in the resulting membrane.

It may be presumed that the gelation in this treatment takes placethrough such mechanism as mentioned below. It appears that, within thebody of the applied filmforrning solution, the ionic atmospheres of theacidic polyelectrolyte and of the basic polyelectrolyte are positionedin a regularly alternate manner and spaced from each other regularly ata constant distance through the action of the inorganic salt and theaction of the dielectric constant of the solvent co-existing therein, sothat a stationary state of balance ismaintained between the ionicatmospheres of the electrolytes. Then the distances between themolecules of the acidic and basic polyelectrolytes are kept so largethat the two polyelectrolytes can be prevented from coprecipitating, asotherwise would take place by crosslinking with each other due to theelectric Coulomb forces, that is, due to the ionic interaction andlinkage through their opposite ionically dissociated groups.

When the dielectric constant value in the solution is now increased bymeans of the above-mentioned treatment, the aforesaid balance can belost with the result that both kinds of the polyelectrolytes havereduced distances between them, and at least portions of their oppositeionically dissociated groups begin to ionically link with each otheruntil the gelation takes place in the solution. At this stage, however,the ions of the inorganic salt as well as the molecules of the water andthe organic solvent (namely, the acetone or dioxane) still stand betweenthe individual partially ionically crosslinked molecules of the acidicand basic polyelectrolytes which thus exhibit only a gelled condition.

The gelled liquid film on the support sheet is subsequently immersed ina bath of water at 10-50C. for a period of 5 minutes to 60 minutes,whereby the inorganic salt and the organic solvent are leached orextracted from the gelled top surface layer. It is thought that, byremoval of the inorganic salt and the organic solvent, the acidic andbasic polyelectrolytes existing in the gel state within the top surfacelayer achieve an enhanced degree of the ionic linkage with each otherand from the continuous and dense layer comprised of the solid polymer.The intermediate layer between the gelled surface and the supportremains a sol solution during the treatment step increasing thedielectric constant in the top layer. But in the water immersiontreatment, the intermediate layer undergoes penetration by water comingthrough the surface layer and the support. The above-described balancemaintained between the polyelectrolyte molecules present in theintermediate layer is then rapidly and severely broken by It i theaction of the penetrating water, with the result that substantially allthe molecules of the acidic and basic polyelectrolytes existing in saidintermediate layer can relatively fast and accurately be reacted witheach other and commence a rapid coprecipitatiomFor the above-mentionedreasons it is believed that the ionically cross-linked polymer withinthe intermediate layer exhibits the structure of higher porosity thanthat of the top surface layer of the film.

The gelled and solidified liquid film carried on the support is thenremoved from the water bath and dried in air at a temperature of 50 to70C. for a period of 3 minutes to 30 minutes, and thereafter rinsed withwater at 10 to 50C. for a period of 5 to 60 minutes. This drying andrinsing operation cycle is then repeated, preferably 5 times to 20times.

By the repeated rinsing operations, the top and intermediate layers ofthe cross-linked polyelectrolyte film are completely freed from theinorganic salts and the organic solvent, and, with the remainingmolecules of the acidic and basic polyelectrolytes still in the solcondition within the intermediate layer, the film is now directly andentirely subjected to the action of the penetrating water, and thus thecourse of the coprecipitation as mentioned above proceeds further in theintermediate reinforcing layer of the film. It is presumed that it isfor these reasons that the cross-linked solid polymer of the acidic andbasic polyelectrolytes could be coprecipitated in the sponge-like orhighly porous structure within the intermediate layer, in contrast withthe tight and continuous structure observed in the top layer. If theinorganic salt is only partly removed from the film through theabove-mentioned aqueous immersion and rinsing treatment, as theremaining trace of the inorganic salt can later deposit as crystals, itwould cause formation of pin-holes in the membrane product.

It is believed that the continuous top surface layer of themolecular-sieve membrane as produced by the process of the presentinvention attains the structure containing uniform micro-pores at anaverage pore diameter adjustable in the range of about l0-120A. forreasons presented by the following explanation. However, the presentinvention is limited in no way to this explanation.

It is known that the individual molecules of a polyelectrolyte in itssol solution take a configuration of a helix coil of several hundreds toseveral thousands angstroms in their dimensions; and also that thedimensions of these helical coils vary depending on the concentrationand ionic strength of any electrolyte possibly co-existing in thepolyelectrolyte solution. Accordingly, it is thought, the ionicatmospheres of the helical polyelectrolyte molecules, within thefilm-forming solution as prepared and used in the process, are subjectedto the influences of the ionic strength of the coexisting inorganic saltand the adjusted dielectric constant of the solvent system, so that suchionic atmospheres of the oppositely charged polyelectrolyte moleculesare arranged alternately and regularly at such a distance that the ionicinteraction and linkage between them cannot take place there.

When the dielectric constant of the solvent system has been increased inthe top surface layer, accordingly, the helical acidic and basicpolyelectrolyte molecules present within the top surface layer muchlyreduce their spacing while keeping their alternate and regulararrangement.

Therefore, the Coulomb forces acting between the dissociated anionic andcationic groups of the acidic and basic polyelectrolytes can be muchenhanced, the acidic and basic polyelectrolyte molecules approach muchcloser to each other, and at least some of the ionically dissociated andoppositely charged acidic and basic polyelectrolyte groups achieve theionic interaction and linkage, while some are left unreacted so that theionic linkage does not occur completely. Thus the gel condition is shownby the top surface layer of the liquid film.

However, it is to be appreciated that the mechanism whereby thedielectric constant increase of the solvent system brings about thegelation in the top surface layer cannot yet be elucidated fully andexactly. Nevertheless, it is certain that the acidic and basicpolyelectrolyte molecules are arranged in a regularly alternate mannerat a regular intermolecular distance even in the gel formed within thetop surface layer by the abovementioned treatment, while the spacesbetween the polyelectrolyte molecules are filled with water molecules,ions of the inorganic salt, and molecules of the organic solvent. Whilethis gelled condition prevails, when the top surface layer is immersedin a bath of water at 10 to 50C. for 5 to 60 minutes, the inorganic saltions and the organic solvent molecules can be extracted into the waterbath from the gelled surface layer and from the spaces between theregularly spaced and alternating acidic and basic polyelectrolytemolecules, leaving spaces into which water molecules now penetrate. Forthis reason and owing to a further increase in the dielectric constantresulting from the removal of the organic solvent, the ionic interactionand ionic linkage at the remaining parts of the ionically dissociatedgroups of the polyelectrolytes further proceeds to completion, butduring this time the described intermolecular spaces are keptsubstantially unchanged. Through this further ionic interaction andlinkage, the much more fully cross-linked polymer achieves the firmlyintegrated three-dimensional network.

In this network, however, it is probable that the individual moleculesof the polyelectrolytes do not show the normal interionic distancebetween their ionically dissociated groups but behave like the freeions; and that the polymer is present in the aqueous gel structure,because a surrounding sheath, consisting of the molecules of the primarybound water and secondary bound water, envelopes each of the ionicallydissociated groups of the polyelectrolytes. The polymer, which has beencross-linked in the above-mentioned way and gained the firmly linkedthree-dimensional network, is insoluble in water and also in variouscommon organic solvents, but it may again be dissolved by the solventsystem used for the particular composition, if desired.

In the polymer network, the acidic and the basic polyelectrolytemolecules have still retained the regularly alternating and spacedarrangement. In addition, the spaces or gaps between the sheaths of thebound water which surround each of the ionically dissociated groups arevery highly reproducible with a width or pore size of a constant valuein the range of about 10-120A., provided that the solvent system of thesame composition is employed, that acidic and basic polyelectrolytes ofthe same natures and the same molecular weights have been used as thestarting material and processed under the same operation conditions.

Though the micro-pores present in the top surface layer of the membraneare so very small that they cannot be observed under an electronmicroscope at 20,000X magnification, it is easy to estimate the valuethickness of 1.1mm, in turn was supported flattened on a glass plate.With aid of a doctor blade, the spreading solution was applied or castat 20C. as a liquid film of 0.5mm. uniform thickness over the top sideof the polyof the a rag p d a The st mat may be propylene sheet. Theapplied liquid film was allowed to made, for example, y comparing theknown molecular stand in open air at 20C. for 30 minutes to evaporatediameters of substances having substantially spherical th tone ordioxane from its top surface, which was molecules which respectively canand cannot pass or h b d t h b m loudy, The liquid film, diffuse throughthe top surface layer and hence through as supported by thepolypropylene sheet on the glass the whole cross-section of themembrane, since molecl was h i d i a w ter bath at 20C for 30 ularWeights are generally correlated to molecular minutes, was removed fromthe water bath, then dried ameters, assuming that the molecules aresubstantially f 5 i t s in ir at 50C,, and rinsed with water forspherical in shape. (For example, a spherical molecular 5 i t s bdipping into the rinsing water at room having a molecular weight on theorder of 1,000,000 is temperature (about 17C.). This cycle of drying andtaken as having a molecular diameter of approximately i in perati wasrepeated 10 times, and thereaf- ZOOAJ g y, Once the average P diameterter the supported molecular sieve membrane so proof the micro-pores inthe top surface layer of a memd eed was removed from the glass plate.

brane product has been estimated, it is easy for those The properties ofthe membrane products so obskilled in the art to predict whether or nota substance tained are tabulated in Table 1 below. They were neuof aknown molecular weight can pass or diffuse tral in nature, as the acidicand basic polyelectrolytes through that membrane product. were used insuch proportions that the number of the The present molecular Sievemembrane ad a taionically dissociable groups present in the acidicpolygeously enables the ultrafiltration and dialysis proelectrolyte usedwas equal to the number of the ioniccesses to be carried out at a higherthroughput, because ally dissociable groups present in the basicpolyelectroit has a high permeability toward water frequently to be lyteused. That is to say, the values of the term: (quanused as the medium,and enables a sharp separation of tity in g. of a polyelectrolyte used)X (specific ionmixed substances or a purification of a crude productexchange capacity in meq./g. of the polyelectrolyte to be effected, asthe micro-pores present in the top used) were equal for both the acidicand basic polyesurface layer are uniform in their pore diameter so thatlectroiytes employed in this case; The concentrations all orsubstantially all the molecules having molecular of the polyelectrolytesare given as the total quantities diameters larger than the said averagepore diameter of the acidic and basic polyelectrolytes dissolved in theare prevented from passing through the membrane. spreading solutions.

The present invention is illustrated by the following The combinedthickness of the continuous top surexamples, to which, however, thepresent invention is face layer and the intermediate reinforcing layerin the not limited. membrane products was found to be about 0.1 to 0.2

' TABLE 1 Top surface Solvent system used Water layer of Totalconpermeation membrane, centration Composition in rate Retention averageof polypercent by weight through of Cytomicropcra Polyelectrolytes usedelectrolytes Dielectric membrane chrome diameter Te in percentAccconstant in ml./cm.= C. in estimated N0. Acidic Basic by weight NaBrtone Water at 20 C. iniinXIO" percent in A. Remarks 7 1. PssNa- PVBTAC2.0 20 1s 02 66.8 2,000 00.0 120 2 PSSNa PVBTAC 3.0 20 1s 62 66.8 2, 20180.0 110 PVB'IAC 5.0 20 18 62 66.8 1,280 95.1 60 Weak membrane.

PVBIAC 7. 0 20 1s 02 00. s 380 09. 5 2o sass: 5-2 2:: 12 2; PVBTAC 4:020 21 59 64:4 "as "s23; "70 i PVBTAC 4.0 20 20 60 05.2 076 85.3 70 ff i'PVBIAC 4.0 20 1s 62 00.2 1,486 see 70 W a d PVBTAC 4.0 20 17 63 07.74,910 Sprea PVBTAC 2.0 38 3 59 78.0 Poor reproducibil- PVBTAC 2.0 so 1000 71.0 200 ity in the PVBTAC 2.0 25 25 60. 0 160 formation of 2g) 51g40 membrane. sass: 2 2 as :22 0 27-2 s2 q. PvB'rAo 4.11s -20 20 00 0512130 0011s 51s imam 10 PVBTAC 3. 0 20 20 05. 2 402 60. 0 20 PVSNa-S'PMVPC-SI 3,0 10 20 66.5 200 40.5 }Wcak membrane. 21 PssNm lMVIC-Su 3.020 20 00 05.2 4&0 51.4

EXAMPLE 1 Referring to Table l:

" Di-electric constants shown are the value as determined for theacetone-water mixture at 20C.

** Water-permeation rate in ml./cm./min. was determined at 20C. with apressure difference of 4Kg./cm. (All the values for the water-permeationrate given in the subsequent examples were measured in the same manner.)

"* Retention of Cytochrome C is a useful standard to determine whetheror not a substance having a vinyl pyridinium chloride with styrene whichhas a n,,,/c value of 14, a molecular weight of 280,000 and anion-exchange capacity of 3.78 meq./g.'

In order to observe the influences of the molecular weights andequivalent ratio in the charged quantities of the polyelectrolytes aswell as the influence of the nature of the inorganic salt added, afurther series of tests was carried out according to the same proceduresQ and under the same conditions as in the abovewas reduced to one-fifthits original volume, determinl0 ing the total amount (C) (in grams) ofCytochrome C mentioned tests. The results obtained are shown in Table 2below.

TABLE 2 .To surface Solvent system used layer of Total con- WaterDermembrane, (imitation Composition in mention rate Retention average ofpolypercent by welght through of cytomicropore Polyelcctrolytes usedelectrolytes Dielectric membrane chrome dimimtor Test i p r nt Acconstant in ml./cm. c. 1" summed No. Acidic Basic by weight NaBr toneWat r at C. mhLXlU percent in A.'"* Remarks 2L... PSSNa PVBTAC h 4.5 2020 on n12 No membrane PSSNa c PVB'IAC l 4.5 20 20 so may; form d(comparatlvc test). 241;... PSSNa PVBTAC h 4.5 20 20 so cs2 840 95.0 6Membrane with a... PSSNa PVB'IAC h 5.0 20 20 so am 600 98.0 55rePmduclzsmu PSSNa PVB'IAC 4.0 '20 20 60 65. 2 No membrane 27-. PSSNaPVBTAC b so in 20 20 so 65.2 160 99.4 60 formed (com- 2a-... PSSNaPVBTAC b 4.0 n 20 20 so 65. 2 220 as. s 55 paratlve test). 29... PSSNaPVB'IAC 4.0 0 2o 20 so 65, 2

Membrane obso PSSH n PVBTAOH 4.1 20 20 so 65.2 020 lined with 31".-.PSSH PVBTAC b 4. 25 2o 20 60 65.2 930 Similar oper 32 PSSCa r PVBTAC b4.5 20 20 50 65.2 420 ties to those f 3a.... PSSH n PMVPC-S a 4. 0 20 2o60 65.2 760 ane 0! geitl N10. 8 of present in the collected filtrate aswell as the total amount (0,) (in grams) of Cytochrome C used in theoriginal solution, and then calculating according to the followingequation:

In all the subsequent examples, the figures for the retention ofCytochrome C or other compounds were evaluated in the same manner asmentioned above. Further referring to Table 1:

Notes:

1. PSSNa (a) means a polystyrene sulfonic acid sodium salt which has a'n /c value of 30, as determined at c 0.5 g./l00ml., a molecular weightof 450,000 and an ion-exchange capacity of 4.74 meq./g. ,[-q,,,/c, hereand throughout, being the reduced specific viscosity of thepolyelectrolyte used].

2. PVBTAC (b) means a polyvinyl benzyl trimethyl ammonium chloride whichhas a 'n /c value of 3.5, as determined at c 0.5 g./l00 ml., a molecularweight of 60,000 and an ion-exchange capacity of 3.38meq./g.

3. "PSSNA (0) means a polystyrene sulfonic acid sodium salt which has a'r /c value of 15, as determined at c 0.5g./l00ml., a molecular weightof 300,000 and an ion-exchange capacity of 4.86meq./g.

4. (d) means that the acetone was replaced by dioxane in this test.

5. (e) means that the NaBr was replaced by CaBr in this test.

6. PVSNa-s (f) means a copolymer of vinyl sulfonic acid sodium salt withstyrene which has a m le value of 8.6, a molecular weight of 160,000 andan ion-exchange capacity of l.50meq./g.

7. PMVPC-S(g)" means a co-polymer of methyl 2- Referring to Table 2:Notes:

8. PSSNA(h) means a. polystyrene sulfonic acid sodium salt which has a n/c value of 2.0, a molecular weight of 30,000 and an ion-exchangecapacity of 4.58meq/g.

9. PVBTAC(i)" means a polyvinyl benzyl trimethyl ammonium chloride whichhas a 17,,lc value of 2.0, a molecular weight of 40,000 and anion-exchange capacity of 3.20 meq/g.

10. (j) means that the PSSNa was used in this test in a quantity twicegreater than the quantity of the PVBTAC by equivalent, and that thefigure given for the concentration of the polyelectrolytes used was intotal percent by weight.

11. (k) means that the PVBTAC was used in this test in a quantity twicegreater than the quantity of the PSSNa by equivalents.

12. (1) means that the NaBr was replaced by NaCl in this test. 13. (m)means that the NaBr was replaced by CaCl,

in this test.

14. (n) means that the NaBr was replaced by Ca(NO in this test.

15. (0) means that the NaBr was replaced by MgCI,

in this test.

l6. PSSl-l(p)" means a polystyrene sulfonic acid which was obtained byconverting the PSSNa(a)" identified in Note 1 of Table 1 into the freeacid by treating with ion-exchange resin.

17. PVBTAOH(q) means a polyvinyl benzyl trimethyl ammonium hydroxide,obtained by converting into the free base the PVBTAC(b)" identified inNote 2 of Table l by treating with ionexchange resin.

l8. PSSCa(r) means a calcium p'olystryene sulfonate, obtained byneutralizing PSSl-l(p)" with calcium hydroxide.

EXAMPLE 2 In this example, a series of tests was carried out to estimatethe retention or permeability of membranes produced in the same manneras in Example 1. in these tests, the retention of various substanceshaving different molecular sizes was determined in the same way asmentioned in Example 1. The results obtained are tabulated in Table 3below.

TABLE s.-ne'riii-irloi i bimRKtfffiifiE OF \maonsimm elm 55"" (Retentionpercent) Hemoglobin Cytochrome (mol weight Tripsin (mol Insulin (molRaifinose (mol Glucose (mol Urea'(mol 67,000 and (mol weight 12,600,weight 5,000, weight 504, weight 180. weight 60. estimated mol weightestimated mol estimated mol estimated mol estimated mol estimated moldiameter 80 A.) 20,000) diameter 45 A.) diameter 30 A.) dimeter 13 A.)diameter 8 A.) diameter 5 A.)

Membrane, percent 100 90. 0 96. 0 70 60 0 0 Membrane, percent. 100 100100 100 75 0 0 Membrane, percent 100 100 100 100 05 33 0 The membranes(a), (b) and (c) tested were produced with PSSNa and the PVBTAC asspecified in the notes 1) and 2) of Table l at the total polyelectrolyteconcentrations of 4.0 percent, 6.5 percent and 7.5 percent by weight,respectively, in a solvent system of the composition of 21 percentNaBr-l8 percent acetone- 62 percent water.

From the tabulated results it may be seen that when the polyelectrolytesand the solvent system are suitably selected, by the present invention,it is easy to produce a membrane which exhibits a practicably high waterpermeability and which is optionally adjustable for the molecular sizesof substances which are retained by or allowed to pass through themembrane.

In a further series of tests the ultrafiltration was carried out usingthe above-mentioned membranes (a), (b) and (c). Thus, an aqueoussolution containing lmg./ml. of Cytochrome C, lmg./ml. of glucose andlmg./ml. of ammonium sulfate was filtered by the membrane (a) in a usualultrafiltration procedure. After the solution was filtered once, thesolution of reduced volume was diluted with water to the originalvolume, and again filtered by means of the same membrane. It was thenobserved that 1 percent of the Cytochrome C, 100 percent of the glucoseand 100 percent of the ammonium sulfate could be passed through themembrane (a) from the solution into the filtrate, so that thepurification of Cytochrome C was achieved.

be removed from 'the solution.

EXAMPLE 3 The molecular-sieve membrane was made in a way similar to thatstated in Example 1, except that the working conditions were changed forthe hereinafter mentioned operations. When the-temperature and periodfor the evaporation of the acetone from the spread liquid film werechanged as described in Table 4 and Table 5, as there tabulated, themembrane products varied in their properties such as water-permeationrate and Retention of Cytochrome C. When the period of thewater-immersion treatment after surface layer gelation and thetemperature of the water during the immersing operation were changed asdescribed in Table 6 and Table 7, the membrane products also varied intheir properties as there tabulated.

When the number of cycles of the drying and waterrinsing operations aswell as the period of the single rinsing operation were changed, themolecular-sieve membrane properties varied as shown in Table 8 and Table9. The PSSNa and the PVBTAC used in this Example 3 were of the samenatures as those specified in the notes 2) and 3) of Table 1 of Example1, respectively. The acidic and basic polyelectrolytes were used at atotal concentration of 4.6 percent by weight, and the solvent systemused had a composition of 20 percent NaBr-ZO percent acetonepercentwater.

TABLE 4 Evaporation temperature C.) 5 10 30 50 Evaporation period (min.)30 30 30 30 30 Water permeation rate (mL/cmfl/min.) 652 10- 480 10-400X10" 260X10- Retention of Cytochrome C (percent) 96 06. 0 91 65. 7Remark 1 Weak membrane.

TABLE 5 Evaporation period (min.) 2 4 0 15 30 00 Evaporation temperatureC.) 20 20 20 20 20 20 20 20 '20 Water permeation rate (ml./cm./min.)550x10" 550Xl0- 548x10 530Xl0-' 520X10- iHOXlO- 470 l0- 460x10 Retentionof Cytochrome 0 (percent) 07. 0 U7. 0 00. R 00. ll 00. 0 04 03. 8 80. i

TABLE 6 Temperature of immersing water C. 5 10 30 50 70 Period of waterimmersion (min.) 20 20 20 20 20 Water permeation rate (mlJemJ/ min.)600x10" 580X10- 510x10" 480x10 Retention of Cytochrome 0 (percent) 97. 096. 8 96. 8 9G. 4

surface of the film for 30 to 120 minutes. The gelation began to takeplace at the top surface of the film. When the above-described treatmentfor increasing the dielectric constant in the supported liquid film hadbeen completed, the water-immersing treatment and the water-rinsing anddrying treatments were carried out in the same way as in Example 1.

There were obtained molecular-sieve membranes which had approximatelythe same properties as those prepared in test No. 8 of Table 1 inExample 1.

EXAMPLE Molecular-sieve membranes were produced in the same way as inExample 1 except that there were used support sheets 1.0mm. thick and ofdifferent materials.

The supports were prepared by forming a non-woven cloth from fibers ofthe different materials as noted in Table below and subsequentlyroll-working the cloth in the same manner as stated for the supportsheet used in Example 1. The molecular-sieve membranes thus preparedwere examined for the adhesion between 'the support layer and theintermediate reinforcing layer. Furthermore, visual and opticalmicroscopic observations were made to estimate the degree to which theintermediate reinforcing layer'of the membrane could cave in the poresof the support layer when the molecular -sieve membrane had beenemployed to carry out the determination of the retention of CytochromeC. The results are tabulated in Table 10 below.

, TABLE 1 Period of water immersion (min) 3 5 6 15 30 60 Temperature ofimmersing water C.) 20 20 20 20 2O 20 20 Water permeation rate(mLIcmfi/min.) 500 10- 510x10 520X1O-4 520X10-4 520 10- 520Xl0- 520x10"Retention of Cytochrome C (percent) 02. 8 93.0 9 8 95. 9 v 96. 8 96. 996. 8

TABLE 8 Number of times for repetition of water-rinsing and dryingoperations. 3 5 10 20 30 Water permeation rate (mL/cmJ/min). 580 l0-540x10 520x10- 51BX10-4 520X10-4 Retention of Cytochrome 0 (percent) 93.2 95. 9 96.8 96. 8 96. 6

TABLE 9 Period of a single water-rinsing operation (min) 3 30 60 90Period for a single drying operation (min.) 20 20 20 20 20 Waterpermeation rate (mL/cmJ/min.) 560 10- 420Xl0' 518x10 518X10-4 517x10-Retention of Cytochrome 0 (percent) 94. 8 96. 8 96. 7 96 7 96. 7

EXAMPLE 4 20 TABLE 10 In the same manner and using the same solvent sys-Average 3:22;; aia g tem composition and the same polyelectrolytes asthose pore anon mg inter. s ecified for the test No. 8 of Exam le 1 afilmdiameter fate mediate l d d d Materials of the in the (mllcm l layerand Degree of ormmg so ution was prepare app 1e an sprea to 25 supportSuppon mm the support caving a film of 0.5mm. depth on the support.While the supported liquid film was held on the glass base plate, ei- 221 33 18 i ther it was introduced into a two-mouthed desiccator,polypropylene 10 Z (LO i+ Slight caving or the space over the liquidfilm was covered as rapidly Polypropylene 5 ,u 2.3 Little cavingPolypropylene 3 u 0. No caving as possible by means of a two moutheddish shaped 3o Polypropylene 1 IL 05 No caving covering. A stream ofair, cooled to about 5 C. and poiypmpylene H H m considerapk completelyfreed from dusts, was blown through one of Cavmg 6-Nylon 38 [1. Completethe mouths into the desiccator or covering onto the top caving surfaceof the film for 60 to minutes. In another 6,6 Nylon 8 H Complete test,the cold cleaned air stream was firstly bubbled 35 C H10 10 s 8 SCthrough a liquid bath of and saturated with the vapor Ce'lulose acetateof dimethyl fonnamide and then blown onto the top "Saran" (polyvinylidene chloride) 38 pi Note: In the above Table 10 the more numerousthe symbol the better is the adhesion; while the symbol means a pooradhesiveness. Thus, means that the molecular-sieve membrane could beformed; but it could not withstand practical use due to an insufficientadhesion between the cross-linked polyelectrolyte layer and the support.-llmeans that a practicable supported membrane was produced but hadrelatively low mechanical strength. +-H- means that the membraneproduced had mechanical strength high enough to be practically appliedowing to the better adhesion between the cross-linked polyelectroyltelayer and the support.

What we claim is:

l. A membrane adapted to be used in ultrafiltration and'dialysisprocesses which comprises:

a. a diffusive continuous top surface layer on the order of 1p, inthickness, which is formed of a polymer produced by ionicallycross-linking, at a ratio of 2:1 to 1:2 by equivalents, a water-solubleacidic polyelectrolyte, or a salt thereof, having a molecular weight offrom about 50,000 to about 700,000 and containing a sufficient number ofsulfonic acid groups to give a pH of up to 2 when said acidicpolyelectrolyte is dissolved in the form of its free acid at aconcentration of 0.6 to 5.4 percent by weight in water, with awater-soluble basic polyelectrolyte, or a salt thereof, having amolecular weight of 50,000 to 700,000 and containing a sufficient numberof quaternary ammonium groups or pyridinium groups to give a pH of atleast when said basic polyelectrolyte is dissolved in the form of itsfree base at a concentration of 0.6 to 5.4 percent by weight in water;

said top layer showing, under electron microscope observation at 5,600xmagnification, a dense and continuous structure containing micro-poresof uniform size having a selected average pore diameter within the rangeof about 10 to 120 A;

b. a reinforcing intermediate layer formed of the ionically cross-linkedpolymer of the same nature as that of the polymer material of andintegral with said top surface layer and which exhibits a structuremicro-porous but rather sponge-like as observed under an electronmicroscope at 5,600x magnification; and

c. a porous support to which the material of said intermediate layer istightly adherent, said support being formed of a polymer selected frompolypropylenes and polyamides and having a porous structure containingpores of an average pore diameter of up to 10 and a water-flow rate ofhigher than 0.5ml./cm. /min. as determined with water at 20C. and at apressure difference of 760 mm Hg;

said top layer, for attainment of said selected average pore diameter,produced under selectively controlled and reproducible conditions as bychoosing suitably the natures, molecular weights and concentrations ofthe polyelectrolytes used as well as the process conditions forionically cross-linking said polyelectrolytes within the abovespecifiedranges.

2. A membrane as claimed in claim 1 in which the acidic polyelectrolyteis selected from the group consisting of polystyrene sulfonic acids andsodium salt and calcium salt thereof.

3. A membrane as claimed in claim 1 in which the acidic polyelectrolyteis a copolymer of styrene with vinyl sulfonic acid, or its alkali metalsalt. v

4. A membrane as claimed in claim 1 in which the basic polyelectrolyteis a polyvinylbenzyl trimethyl ammonium halide.

5. A membrane as claimed in claim 1 in which the basic polyelectrolyteis a copolymer of styrene with a methyl-Z-vinyl pyridinium halide.

6. A membrane as claimed in claim 1 in which the top layer and theintermediate layer are formed of an ionically cross-linked polymericreaction product of a polystyrene sulfonic acid with a polyvinylbenzyltrimethyl ammonium hydroxide.

7. A membrane as claimed in claim 1 in which the top layer and theintermediate layer of the membrane are formed of an ionicallycross-linked polymeric reaction product of a polystyrene sulfonic acidwith a polyvinylbenzyl trimethyl ammonium halide.

8. A membrane as claimed in claim 1 in which the top layer and theintermediate layer are formed of an ionically cross-linked polymericreaction product of an alkali or alkaline earth metal salt of apolystyrene sulfonic acid with a polyvinylbenzyl trimethyl ammoniumhalide.

9. A membrane as claimed in claim 1 in which the top layer and theintermediate layer are formed of an ionically cross-linked polymericreaction product of a polystyrene sulfonic acid with apolymer-2-vinyl-pyridinium halide.

10. A membrane as in claim 1 in which the top layer and the intermediatelayer are formed of an ionicallycross-linked polymeric reaction productof an alkali or alkaline earth metal salt of a polystyrene sulfonic acidwith a polymethyl-Z-vinyl-pyridinium halide.

11. A membrane as claimed in claim 1 in which the top layer and theintermediate layer are formed of an ionically cross-linked polymericreaction product of an alkali metal salt of a styrene/vinyl sulfonicacidcopolymer with a polyvinylbenzyl trimethyl ammonium halide.

12. A membrane as claimed in claim 1 in which the top layer and theintermediate layer are formed of an ionically cross-linked polymericreaction product of an alkali metal salt of a styrene/vinyl sulfonicacid copolymer with a polymethyl-Z-vinyl pyridinium halide.

13. A membrane as claimed in claim 1 in which the top layer and theintermediate layer of the membrane have a total thickness on the orderof 0.1-0.2mm.

14. A membrane as claimed in claim 1 in which the porous support isderived from a sheet of a non-woven cloth of fibers of a materialselected from the polypropylenes and polyamides, said sheet smoothenedat its faces by compressing the cloth in the sintered state betweenapair of rollers to .a thickness of about 0.1 to 2.0mm so as to impartthereto a porosity such that the average pore diameter of the sheet isup to 10p. but the flow rate of water passing through the sheet amountsto a value of higher than 0.5ml./cm. /rnin. at 20C. and at a pressuredifference of 760mm Hg.

15. A process for the production of a membrane as claimed in claim 1,which comprises:

i. preparing an organic solvent-water type solvent system by dissolvingat least one of sodium bromide, sodium iodide, calcium chloride, calciumbromide and calcium nitrate to a concentration of 10 to 35 percent byweight of the resulting solvent system into a mixture of acetone andwater or into a mixture of dioxane and water mixed together at such aratio as to give a dielectric constant value of 50 to 78 when determinedat 20C., .,dissolving, into said solvent system to a total concentrationof 2 to 8 percent by weight of the solvent system at a ratio of 2:1 to1:2 by equivalents, a water-soluble, acidic polyelectrolyte, or a saltthereof, having a molecular weight of 50,000 to 700,000 and containing asufficient number of sulfonic acid groups to give a pH of up to 2 wheresaid acidic polyelectrolyte is dissolved in the form of its free acid inwater at a concentration of 0.6 to 5.4 percent by weight and awater-soluble, basic polyelectrolyte, or a salt thereof, having amolecular weight of 50,000 to 700,000 and containing a sufficient numberof quaternary ammonium groups or pyridinium groups to give a pH of atleast 10 where said basic polyelectrolyte is dissolved in thev form ofits free base in water at a concentration of 0.6 to.

5.4 percent by weight,

iii. spreading the resulting solution or sol solution of thepolyelectrolyte as a thin uniform liquid film onto a support sheet madeof a polypropylene or polyamide, said support sheet exhibiting a porousstructure containing pores of an average pore diameter of up to 10 butshowing a water-flow rate higher than 0.5 ml./cm./min. as determinedwith water at 20C. and under a pressure difference of 760mm Hg,

iv. treating the film at a temperature of from 10 to 50C..to increasethe dielectric constant value .of

the solvent system in the top surface layer of said thin liquid film,

v. continuing this treatment for a period of from to 120 minutes tobring about gelation in the top surface layer of the liquid film whilethe intermediate layer positioned between said top surface layer and thesupport is maintained in the sol condition,

vi. immersing the surface-gelled liquid film with the support sheet intoa bath of water at to 50C. for a period of 5 to 60 minutes to remove atleast parts of the inorganic salt and of the organic solvent from thefilm to solidify the entire film thickness,

vii. withdrawing the solidified film with the support from the waterbath,

viii. drying the composite of the solidified film and the support sheetat a temperature of 50 to 70C. for a period of 3 to 30 minutes,

ix. then rinsing said composite with water at from 10 to 50C. for aperiod of 5 to 60 minutes, and

x. repeating the above cycle of drying and rinsing operations until thesolidified film on the support is completely free of the inorganic saltand of the organic solvent,

xi. the natures, molecular weights and concentrations of thepolyelectrolytes used, the natures and numbers of the ionicallydissociable groups of the poly electrolytes, the natures andconcentrations of the inorganic salts used etc. as well as the ratio ofthe organic solvent to water and the process conditions for gelation andwater-immersion during the formation of the gelled and solidified filmof the cross linked polyelectrolyte polymers, being selectively andreproducibly controlled within the abovereferred ranges so as to producea diffusive continuous top surface layer of said solidified film havinga dense and continuous structure containing micropores of uniform sizehaving said selected average pore diameter within the range of about 10to 120 A.

16. A process as claimed in claim in which said polyelectrolytes arerespectively a polystyrene sulfonic acid, and a polyvinylbenzyltrimethyl ammonium hydroxide.

17. A process as claimed in claim 15 in which said polyelectrolytes arerespectively a polystyrene sulfonic acid, and a polyvinylbenzyltrimethyl ammonium halide.

18. A process as claimed in claim 15 in which said polyelectrolytes arerespectively an alkali or alkaline earth metal salt of a polystyrenesulfonic acid and a polyvinylbenzyl trimethyl ammonium halide.

19. A process as claimed in claim 15 in which said polyelectrolytes arerespectively a polystyrene sulfonic acid, and a poIymethyl-Z-vinylpyridinium halide.

20. A process as claimed in claim 15 in which said polyelectrolytes arerespectively an alkali or alkaline earth metal salt of a polystyrenesulfonic acid and a polymethyl-2-vinyl-pyridihium halide.

21. A process as claimed in claim 15 in which said polyelectrolytes arerespectively an' alkali metal salt of a styrene/vinyl sulfonic acidcopolymer, and a polyvinyl benzyl trimethyl ammonium halide. Y

22. A process as claimed in claim 15 in which said polyelectrolytes arerespectively an alkali metal salt of a styrene/vinyl sulfonic acidcopolymer, and a polymethyl-Z-vinyl pyridinium halide.

' 23. A process as claimed in claim 15 in which said polyelectrolytesare present in a ratio of 1:1 by equivalents.

24. A process as claimed in claim 15 in which the solvent system is amixture of sodium bromide, acetone and water containing 10 to 30 percentby weight of sodium bromide, l to 25 percent by weight of acetone andthe balance water, and showing a dielectric constant value of 50 to 78as determined at 20C.

25. A process as claimed in'claim 15 in which the treatment forincreasing the dielectric constant value of the solvent system in thesaid top layer is effected by allowing the supported liquid film tostand in air at 10 to 50C for a period of 5 to minutes during which theorganic solvent is evaporated from the top surface of the liquid film.

26. A process as claimed in claim 15 in which the treatment forincreasing the dielectric constant value of the solvent system in thetop layer of the liquid film applied on the support sheet is effected bykeeping the supported liquid film in air at a temperature of 10 to 50C.and then blowing a stream of an inert gas at a temperature on the orderof 5C. onto the top surface of the liquid film for a period of 5 to 120minutes.

27. A process as claimed in claim 15 in which the treatment forincreasing the dielectric constant value of the solvent system in theapplied liquid film is carried out by keeping the supported film in airat 10 to 50C. and then blowing a dimethyl 'formamide vaporsaturated airstream at 5C. onto the top surface of the liquid film for a period offrom 5 to 120 minutes.

f f i i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3 737 045 Dated 7 June 5 1973 Inventor s) Koichi Hashimoto et 1 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

on the cover sheet [73] Assignee, "Uivac Corporation" should read UlvacCorporation Column 5, line 64, "usual to practice that the dialysis andeven the in be not" should read usual in'practice that the dialysis andeven the ultrafiltration be not Column 15, the fifth line below Table 2(C "C()] x 100" should read [(C -C)/C x 100 Column 15,. fi th line frombottom "6. "PVSNa-s (f)" should read 6. "PVSNa--S(f)" In Table I.(crossing columns 13-14); in the ninth table column, n "in ml ./cm. min.X l0 should read in ml./cm. /min. x 10 in the last table column "w kmembrane", which is bracketed to the data rows of- Tests No. l-5, isapplicable only to "TestNo. l "Highly viscous solution somewhatdifficult to spread." which is bracketed to the data rows of Tests No..6-10, is applicable only to Test N0. 6 "Poor reproducibility: in theformation of the membrane", which is bracketed to the data rows forTests Nos. 11-14, is applicable only to Test No. ll "No membrane formed(comparative test)." which is bracketed to the data rows of Test Nol5-l8, is applicable only to Test No. 15 in the sixth table column(under "Acetone)", the entry for Test No. 15, for "c 27" should read d27 Column 13, line adjacent printed line number "60", "Table 1 below"should read Table 1 above,

In "Table 2", crossing columns 15-16, in the second column entry at TestNo. 30, ."PVBTAOHg" should read PVBTAOHq in the last table column,headed "Remarks" "No membrane formed (comparative test)." which isbracketed to the data rows for Test Nos. 26-29, is applicable only toTest No. 26 and to Test No. 29; "Membrane obtained. .of Table 1", whichis bracketed to the data rows for Test Nos. 30-33, is applicable only toTest Nos.

30-31 In "Table 7" crossing columns 19-20, at the third 1 uscoMM-Dcwan-Poo i LLS. GOVERNMENT PRINTING OFFICE: 19! 0-36 -334,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,737,045 Dated June 5, 1973 Inventor) j Koichi Hashimoto f p It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

entry in the row for "Water Permeation rate-", in the table column forthe "6" minute immersion, "520 x 10' should read 510'): 10' Table 9,crossing columns 19-20, at the second entry in the row for the "Waterpermeation rate" in the table column for f he "5" minute rinsing, "420x:".l0'- should read 520 x 10, Table 10, patent column'ZO, under thefourth table column heading "Adhesion...support",

in the third row (first Polypropylene .OCCUIGII'ICG'V) the symbol "1+"should read-- in the last three rows (for cellulose, cellulose acetate,and "Saran") respectively a minus sign (-J should appear. 1

Signed and sealed this 9th day'of July 1974.

(SEAL) Attest':

MCCOY M. GIBSON, JR. AR H N Attesti Offi v Commissioner of Patents I-ORM30-1050 (0.69) i USCOMM-DC cove-p09 1* [1.5. GOVERNMENT FRIIH'ING OFFICE"I! 0-366-35.

2. A membrane as claimed in claim 1 in which the acidic polyelectrolyteis selected from the group consisting of polystyrene sulfonic acids andsodium salt and calcium salt thereof.
 3. A membrane as claimed in claim1 in which the acidic polyelectrolyte is a copolymer of styrene withvinyl sulfonic acid, or its alkali metal salt.
 4. A membrane as claimedin claim 1 in which the basic polyelectrolyte is a polyvinylbenzyltrimethyl ammonium halide.
 5. A membrane as claimed in claim 1 in whichthe basic polyelectrolyte is a copolymer of styrene with amethyl-2-vinyl pyridinium halide.
 6. A membrane as claimed in claim 1 inwhich the top layer and the intermediate layer are formed of anionically cross-linked polymeric reaction product of a polystyrenesulfonic acid with a polyvinylbenzyl trimethyl ammonium hydroxide.
 7. Amembrane as claimed in claim 1 in which the top layer and theintermediate layer of the membrane are formed of an ionicallycross-linked polymeric reaction product of a polystyrene sulfonic acidwith a polyvinylbenzyl trimethyl ammonium halide.
 8. A membrane asclaimed in claim 1 in which the top layer and the intermediate layer areformed of an ionically cross-linked polymeric reaction product of analkali or alkaline earth metal salt of a polystyrene sulfonic acid witha polyvinylbenzyl trimethyl ammonium halide.
 9. A membrane as claimed inclaim 1 in which the top layer and the intermediate layer are formed ofan ionically cross-linked polymeric reaction product of a polystyrenesulfonic acid with a polymer-2-vinyl-pyridinium halide.
 10. A membraneas in claim 1 in which the top layer and the intermediate layer areformed of an ionically-cross-linked polymeric reaction product of analkali or alkaline earth metal salt of a polystyrene sulfonic acid witha polymethyl-2-vinyl-pyridinium halide.
 11. A membrane as claimed inclaim 1 in which the top layer and the intermediate layer are formed ofan ionically cross-linked polymeric reaction product of an alkali metalsalt of a styrene/vinyl sulfonic acid copolymer with a polyvinylbenzyltrimethyl ammonium halide.
 12. A membrane as claimed in claim 1 in whichthe top layer and the intermediate layer are formed of an ionicallycross-linked polymeric reaction product of an alkali metal salt of astyrene/vinyl sulfonic acid copolymer with a polymethyl-2-vinylpyridinium halide.
 13. A membrane as claimed in claim 1 in which the toplayer and the intermediate layer of the membrane have a total thicknesson the order of 0.1-0.2mm.
 14. A membrane as claimed in claim 1 in whichthe porous support is derived from a sheet of a non-woven cloth offibers of a material selected from the polypropylenes and polyamides,said sheet smoothened at its faces by compressing the cloth in thesintered state between a pair of rollers to a thickness of about 0.1 to2.0mm so as to impart thereto a porosity such that the average porediameter of the sheet is up to 10 Mu but the flow rate of water passingthrough the sheet amounts to a value of higher than 0.5ml./cm.2/min. at20*C. and at a pressure difference of 760mm Hg.
 15. A process for theproduction of a membrane as claimed in claim 1, which comprises: i.preparing an organic solvent-water type solvent system by dissolving atleast one of sodium bromide, sodium iodide, calcium chloride, calciumbromide and calcium nitrate to a concentration of 10 to 35 percent byweight of the resulting solvent system into a mixture of acetone andwater or into a mixture of dioxane and water mixed together at such aratio as to give a dielectric constant value of 50 to 78 when determinedat 20*C., ii. dissolving, into said solvEnt system to a totalconcentration of 2 to 8 percent by weight of the solvent system at aratio of 2:1 to 1:2 by equivalents, a water-soluble, acidicpolyelectrolyte, or a salt thereof, having a molecular weight of 50,000to 700,000 and containing a sufficient number of sulfonic acid groups togive a pH of up to 2 where said acidic polyelectrolyte is dissolved inthe form of its free acid in water at a concentration of 0.6 to 5.4percent by weight and a water-soluble, basic polyelectrolyte, or a saltthereof, having a molecular weight of 50,000 to 700,000 and containing asufficient number of quaternary ammonium groups or pyridinium groups togive a pH of at least 10 where said basic polyelectrolyte is dissolvedin the form of its free base in water at a concentration of 0.6 to 5.4percent by weight, iii. spreading the resulting solution or sol solutionof the polyelectrolyte as a thin uniform liquid film onto a supportsheet made of a polypropylene or polyamide, said support sheetexhibiting a porous structure containing pores of an average porediameter of up to 10 Mu but showing a water-flow rate higher than 0.5ml./cm.2/min. as determined with water at 20*C. and under a pressuredifference of 760mm Hg, iv. treating the film at a temperature of from10* to 50*C. to increase the dielectric constant value of the solventsystem in the top surface layer of said thin liquid film, v. continuingthis treatment for a period of from 5 to 120 minutes to bring aboutgelation in the top surface layer of the liquid film while theintermediate layer positioned between said top surface layer and thesupport is maintained in the sol condition, vi. immersing thesurface-gelled liquid film with the support sheet into a bath of waterat 10* to 50*C. for a period of 5 to 60 minutes to remove at least partsof the inorganic salt and of the organic solvent from the film tosolidify the entire film thickness, vii. withdrawing the solidified filmwith the support from the water bath, viii. drying the composite of thesolidified film and the support sheet at a temperature of 50* to 70*C.for a period of 3 to 30 minutes, ix. then rinsing said composite withwater at from 10* to 50*C. for a period of 5 to 60 minutes, and x.repeating the above cycle of drying and rinsing operations until thesolidified film on the support is completely free of the inorganic saltand of the organic solvent, xi. the natures, molecular weights andconcentrations of the polyelectrolytes used, the natures and numbers ofthe ionically dissociable groups of the polyelectrolytes, the naturesand concentrations of the inorganic salts used etc. as well as the ratioof the organic solvent to water and the process conditions for gelationand water-immersion during the formation of the gelled and solidifiedfilm of the cross-linked polyelectrolyte polymers, being selectively andreproducibly controlled within the above-referred ranges so as toproduce a diffusive continuous top surface layer of said solidified filmhaving a dense and continuous structure containing micro-pores ofuniform size having said selected average pore diameter within the rangeof about 10 to 120 A.
 16. A process as claimed in claim 15 in which saidpolyelectrolytes are respectively a polystyrene sulfonic acid, and apolyvinylbenzyl trimethyl ammonium hydroxide.
 17. A process as claimedin claim 15 in which said polyelectrolytes are respectively apolystyrene sulfonic acid, and a polyvinylbenzyl trimethyl ammoniumhalide.
 18. A process as claimed in claim 15 in which saidpolyelectrolytes are respectively an alkali or alkaline earth metal saltof a polystyrene sulfonic acid and a polyvinylbenzyl trimethyl ammoniumhalide.
 19. A process as claimed in claim 15 in which saidpolyelectrolytes are respectively a polystyrene sulfonic acid, and apolymethyl-2-vinyl pyridinium halide.
 20. A process as claimed in claim15 in which said polyelectrolytes are respectively an alkali or alkalineearth metal salt of a polystyrene sulfonic acid and apolymethyl-2-vinyl-pyridinium halide.
 21. A process as claimed in claim15 in which said polyelectrolytes are respectively an alkali metal saltof a styrene/vinyl sulfonic acid copolymer, and a polyvinyl benzyltrimethyl ammonium halide.
 22. A process as claimed in claim 15 in whichsaid polyelectrolytes are respectively an alkali metal salt of astyrene/vinyl sulfonic acid copolymer, and a polymethyl-2-vinylpyridinium halide.
 23. A process as claimed in claim 15 in which saidpolyelectrolytes are present in a ratio of 1:1 by equivalents.
 24. Aprocess as claimed in claim 15 in which the solvent system is a mixtureof sodium bromide, acetone and water containing 10 to 30 percent byweight of sodium bromide, 1 to 25 percent by weight of acetone and thebalance water, and showing a dielectric constant value of 50 to 78 asdetermined at 20*C.
 25. A process as claimed in claim 15 in which thetreatment for increasing the dielectric constant value of the solventsystem in the said top layer is effected by allowing the supportedliquid film to stand in air at 10* to 50*C for a period of 5 to 120minutes during which the organic solvent is evaporated from the topsurface of the liquid film.
 26. A process as claimed in claim 15 inwhich the treatment for increasing the dielectric constant value of thesolvent system in the top layer of the liquid film applied on thesupport sheet is effected by keeping the supported liquid film in air ata temperature of 10* to 50*C. and then blowing a stream of an inert gasat a temperature on the order of 5*C. onto the top surface of the liquidfilm for a period of 5 to 120 minutes.
 27. A process as claimed in claim15 in which the treatment for increasing the dielectric constant valueof the solvent system in the applied liquid film is carried out bykeeping the supported film in air at 10* to 50*C. and then blowing adimethyl formamide vapor-saturated air stream at 5*C. onto the topsurface of the liquid film for a period of from 5 to 120 minutes.